1. Field of Invention
The invention relates generally to antennas for radiating and/or receiving electromagnetic energy, and specifically in one aspect to an antenna in which the radiators are conductor coatings of a dielectric chip; the chip may be, e.g., mounted on a circuit board of a radio device, wherein the circuit board is a part of the antenna structure.
2. Description of Related Technology
In small-sized radio devices, such as mobile phones, the antenna or antennas are preferably placed inside the cover of the device, and naturally the intention is to make them as small as possible. An internal antenna has usually a planar structure so that it includes a radiating plane and a ground plane below it. There is also a variation of the monopole antenna, in which the ground plane is not below the radiating plane but farther on the side. In both cases, the size of the antenna can be reduced by manufacturing the radiating plane on the surface of a dielectric chip instead of making it air-insulated. The higher the dielectricity of the material, the smaller the physical size of an antenna element of a certain electric size. The antenna component becomes a chip to be mounted on a circuit board. However, such a reduction of the size of the antenna entails the increase of losses and thus a deterioration of efficiency.
FIG. 1 shows a chip antenna known from the publications EP 1 162 688 and U.S. Pat. No. 6,323,811, in which antenna there are two radiating elements side by side on the upper surface of the dielectric substrate 110. The first element 120 is connected by the feed conductor 141 to the feeding source, and the second element 130, which is a parasitic element, by a ground conductor 143 to the ground. The resonance frequencies of the elements can be arranged to be different in order to widen the band. The feed conductor and the ground conductor are on a lateral surface of the dielectric substrate. On the same lateral surface, there is a matching conductor 142 branching from the feed conductor 141, which matching conductor is connected to the ground at one end. The matching conductor extends so close to the ground conductor 143 of the parasitic element that there is a significant coupling between them. The parasitic element 130 is electromagnetically fed through this coupling. The feed conductor, the matching conductor and the ground conductor of the parasitic element together form a feed circuit; the optimum matching and gain for the antenna can then be found by shaping the strip conductors of the feed circuit. Between the radiating elements, there is a slot 150 running diagonally across the upper surface of the substrate, and at the open ends of the elements, i.e. at the opposite ends as viewed from the feeding side, there are extensions reaching to the lateral surface of the substrate. By means of such design, as well by the structure of the feed circuit, it is aimed to arrange the currents of the elements orthogonally so that the resonances of the elements would not weaken each other.
A drawback of the above described antenna structure is that in spite of the ostensible optimization of the feed circuit, waveforms that increase the losses and are effectively useless with regard to the radiation produced by the device are created in the dielectric substrate. The efficiency of the antenna is thus comparatively poor and not satisfactory. In addition, there is significant room for improvement if a relatively even radiation pattern, or omnidirectional radiation, is required.